As telecommunication data transmission is increasingly reliant on packet-based networks (e.g. Ethernet, ATM, PSN), robust methods for time and frequency synchronization between certain nodes of these networks are required. By synchronization, a way of distributing common time and frequency references to networked devices (or nodes), in order to align their time and frequency scales, is meant.
For example, Wireless networks reveal such requirement at the air/radio interface with usual values below 50 ppb (parts per billion) and 1 μs (micro second) targets, respectively, for frequency and time accuracies. At the network level (e.g. at the input of the radio Base Station), a synchronization accuracy around 16 ppb or even less is usually quested.
In this regard, timestamp-based synchronization protocols are widely adopted, typically as the IEEE 1588V2 standard (also known as PTPV2 for Precision Time Protocol Version 2) or the IETF Network Time Protocol Version 4 (NTPV4). Such synchronization protocols are in charge of aligning the time (respectively the frequency) of at least a network node, commonly designated as Slave, to the time (respectively to the frequency) of another node, commonly designated as Master. To that end, these synchronization protocols are constrained to measure the path delay (or even the roundtrip delay) introduced by the network path linking the source (Master/Slave) to the destination (Slave/Master).
Hence, synchronization protocol performances may be directly affected by Packet Delay Variation (PDV) along the Master-to-Slave and/or Slave-to-Master network path. Different traffic load conditions, the number of hops (e.g. switches, routers), networks links capacities, route changes and other hardware taking place between the Master clock and the Slave clock are mainly at the origin of such delay/Packet Delay Variation.
Typically, with a well defined floor delay, the larger the PDV, the more pronounced noising effect on synchronization protocol performances. Accordingly, PDV over packet-based networks has a direct impact on performances of time and frequency packet transfer protocols.
The key performance factor to accurately assure compliance to packet-based network synchronization requirements is the whole management of PDV, in other words an End-to-End PDV management.
However,                either the PDV per network hop/segment management, followed by End-to-End PDV deduction—even if PDV is not strictly additive—;        or the PDV measurement of all (Slave, Path, Master) combinations;require the deployment of specific resources and monitoring tools within the network in order to measure all End-to-End (E2E) PDVs or network hops. Moreover, such static and conservative approaches are not well-suited to ubiquitously dynamic environments as Packet-Switched Networks (PSN). Indeed, the network traffic (especially for the mobile backhaul with 3G and 4G evolutions) is continuously increasing, considering that demanding increase in bandwidth meets targeted services (Triple Play Service Delivery Architecture). This traffic increase has a direct impact on PDV and, consequently, on the per-PDV-domain synchronization topology organization.        
This means that packet-based networks operators have to regularly                monitor their network traffic load and the related induced PDV; and accordingly        review the synchronization topology organization.        
Such burden is a complex task, as well as it requires an important operating expense proportional to the complexity of the synchronization topology.
A “free-PDV” IEEE1588V2 topology, commonly known as IEEE 1588V2P2P TC (Peer-to-Peer Transparent Clocks) or IEEE 1588V2 E2E TC (End-to-End Transparent Clocks), permits to correct the delay in one-way direction in real-time. Nevertheless, this approach requires the deployment of a plurality of complex hardware all over the network nodes within the synchronization topology which may discourage its adoption over wide or deep synchronization topologies.
Carefully monitoring PDV, experienced by timing packets, by applying relevant Quality of Service (QoS) policies on the network (e.g. by prioritizing timing flows) may respond to the evoked difficulties. However, QoS does not permit to suppress completely the PDV as ultimately there is always competition between PTP flows within the premium queue and there are always competitions between PTP packets and best-effort or data traffic when it comes closed to the transmission medium. Moreover, an end-to-end QoS management is not always available for multiple reasons as networks heterogeneity or QoS policies variability per networks segments. Moreover, this approach would engender difficult situations for networks operators, especially in high traffic load situations (e.g. impossibility to redirect the traffic, traffic equity). But above all, the relationship between QoS rules and PDV ranges has to be clearly demonstrated before any deployment of such solution.
One object of the present invention is therefore to provide a migration way for networks operators targeting an autonomous management of their synchronization network while keeping a full control of it.
Another object of the present invention is to jointly allow monitoring and controlling synchronization performances by measuring/assessing the PDV over synchronization network.
Another object of the present invention is to automatically achieve best performances of timestamp-based synchronization protocols over a given synchronization topology.
Another object of the present invention is to provide a global PDV management over a synchronization topology with a given Slave synchronized to a Master.
Another object of the present invention is to reduce the impact of PDV on the accuracy of time and frequency packet transfer protocols.
Another object of the present invention is to enhance frequency and time distribution protocol performances over wide-area Ethernets, Packet-Switched Networks (PSNs) and Circuit Emulation Services (CES).
Another object of the present invention is to support the deployment, over distributed packet-based networks, applications that require precise time and/or frequency synchronization.
Another object of the present invention is to automatically identify and configure, upon operator acknowledgement, the relevant synchronization topology, whatever the traffic load scenario.
Another object of the present invention is to provide an automatic timekeeping within distributed clock synchronization in packet-based networks.
Another object of the present invention is to provide a per-PDV domain organization of synchronization topology in automatic and self management manner.
Another object of the present invention is to optimize the signaling message rate used for PDV assessment, while keeping high PDV confidence range.